1. Field of the Invention
This invention relates to a method of and apparatus for controlling the position of a grading implement relative to a datum by utilizing a non-contact distance sensor. More particularly, the invention relates to a method of and apparatus for controlling the position of grading implement comprising the steps of beginning a measurement cycle with a command generated by a central processing unit to cause a driver to power a sonic transducer, the transducer producing a short burst of ultrasonic waves which are sensed by a receiver which will detect signals returning both from a datum and from a compensation target, the central processing unit thereafter recording the number of clock cycles that have occurred between the beginning of the measurement cycle and the reception of the return signals, thereby allowing the central processing unit to logically determine if a temperature compensation target, reference target, and measured surface signal has been received, and to calculate the distance to the measured surface.
2. Description of Related Technology
Motor graders, bulldozers, pavers, road construction vehicles, earth moving vehicles and certain agricultural equipment are often called upon to either produce or sense a graded surface that follows a predetermined datum. In areas of new construction, for example, the datum might be defined by a string line supported by stakes placed adjacent to the path to be graded. In areas of reconstruction, the datum might be defined by a preexisting curb or pavement surface. On the other hand, the crop gathering header of a harvester combine, for example, travels as close to the nominally level surface of the ground as possible in order to gather and harvest low-lying feed crops such as soybeans.
Various devices have been used on earth moving and agricultural vehicles to position the grading or harvesting implements to obtain the desired graded surface or crop gathering header profile. The grading implements of these vehicles are usually positioned by hydraulic cylinders that are coupled to mechanisms that support the grading implements. A typical positioning device includes a datum sensing device that is mounted on the grading implement and that senses the position of the datum relative to the implement, and a control device that signals the hydraulic cylinders to reposition the implement accordingly. The cutting mechanism carried by the crop gathering header of a harvester combine frequently encounters surface rises and depressions in the contour of the terrain which cause the header to bounce and occasionally dig the crop cutting mechanism into the ground. The header should be maintained am a minimum position above the ground in order to avoid picking up stones and running the cutting mechanism into the ground. Header height controls are known for the purpose of detecting ground contour variations and signalling them to hydraulically or electrically operated power devices which then adjust the height of the header above the ground, with sufficient lead time so as to prevent the cutting mechanism from digging into the ground.
In the context of paving applications, the datum must be sensed by a device such as a string follower, which is used where the datum is defined by a string line that is suspended a constant distance above the desired graded surface. A sensor unit is attached to the side of the grading implement nearest the string line, and a pivotablee wand extends from the sensor unit and touches the string line. The sensor is responsive to the rotational position of the wand as an indication of the position of the implement relative to the string datum. The wand is often spring loaded against the string line to ensure contact. One drawback to the use of the string follower is that if the spring force is excessive, or if the string line is loosely strung, then the spring force of the wand can displace the string line from its intended position and thereby introduce grading errors. Another drawback is that the operator must stop grading and get out of the cab in order to set the wand onto the string line. Another drawback is that if the wand falls off of the string line, then the sensor indicates a large positional error and tries to correct the position of the implement accordingly, thus causing gouges or other discontinuities in the graded surface. Still another drawback is that the wand mechanism typically has limited adjustability which restricts the location of the string line relative to the desired surface.
In an agricultural context, known header height controls have a "dead band" within which the control is disabled when the header height is between the "raise" and "lower" dead band limits at which the height sensors actuate the electrohydraulic power device to respectively raise and lower the header, and some controls also have means to vary the width of such a dead band. However, most known header height controls require that the operator stop the combine and climb down from the cab in order to vary the dead band. Further, it is difficult to adjust both dead band and height to obtain optimum settings in most known header height controls.
Other datum sensing devices include wheels and skids, which are useful where the datum is defined by a preexisting curb or previously graded surface. A sensor unit is usually mounted to the side or rear of the grading implement, and a projecting arm pivots downward to place the wheel or skid on the datum surface. The sensor unit responds to the rotational position of the arm as a measure of the position of the implement relative to the datum surface. One major drawback to the use of wheels and skids is that they are typically designed for forward movement of the vehicle, so that in order to allow the vehicle to back up, the wheel or skid must be lifted. If the vehicle backs up without lifting such a wheel or skid, then the mounting mechanism may bend or break.
Known header height controls include a four bar mechanical linkage including a turnbuckle which actuates a rotatable cam which operates "raise" and "lower" limit switches that derive electrical command signals for "raise" and "lower" solenoids. The header height set point represents the midpoint in the travel of the rotatable cam between the "raise" dead band limit at which the cam operates the "raise" limit switch and the "lower" dean band limit at which it operates the "lower" limit switch. The header height set point is adjusted by the operator getting out of the cab and changing the length of the turnbuckle in the four bar mechanical linkage and similarly the dead band is adjusted by changing the position of the hole in a radial link of a four bar linkage in which the turnbuckle is engaged.
Examples of other Height sensors include U.S. Pat. No. 3,704,574, issued to Gardner, which discloses a mechanical system associated with header height sensors for moving a valve stem in or out, depending upon the movement of the header height sensor above or below a predetermined setting.
U.S. Pat. No. 3,851,451, issued to Agness et al., discloses an automatic height control system having a number of individually operable ground followers or height sensors which are associated with switches electrically connected to a solenoid valve for controlling the vertical movement of a header of a crop gathering unit.
U.S. Pat. No. 4,136,508, discloses an automatic header height control for the crop gathering header of a combine which permits the operator to change the desired height setting from the operator's platform without stopping the combine and to adjust the dead band within which the header can move from a height setting without generating a correction signal from the operator's platform and do so without stopping the combine. The dead band adjust mechanism includes a potentiometer.
U.S. Pat. No. 3,736,486 discloses a dead band control circuit and a servo control system for a digital serve motor control and drive system which is particularly useful in controlling a servo motor used in editing images in a motion picture film to videotape transfer system. The dead band control circuit includes an adjustable potentiometer whereby the dead band can be adjusted accurately by adjusting the wiper arm of the potentiometer.
U.S. Pat. No. 3,083,327, issued by Byloff, discloses a dead band control system in which, at the edge of the modulated band, full power pulses are generated for moving a valve motor in a clockwise direction or in a counterclockwise direction to move a valve element. Smaller signals which normally would have little effect are utilized to provide control current pulses of full power and varying pulse duration to extend the circuit operation past and into the dead band zone. These control current pulses provide "inching" operation of the valve motor.
U.S. Pat. No. 4,437,295, issued to Rock, discloses a pulsing automatic header height control which provides a very small dead band zone with no control within the dead band zone, by providing a system which is utilized for controlling the height of a header as opposed to a butterfly valve element in an air duct.
U.S. Pat. No. 4,573,124, issued to Seiferling, discloses a height control in which the distance to the ground is determined and electrical signals corresponding to the distance are generated. A control circuit receives the signals and provides "up" and "down" pulses. A monitoring and controlling circuit receives the "up" and "down" pulses and transmits them to a power device for actuating an incremental step up or step down for each pulse. The monitor determines whether a preselected number of consecutive "down" pulses occur without an "up" pulse between them and within a short predetermined time period and, if not, actuates a circuit disabling and overriding the power device only when a preselected number of "down" pulses occurs within a predetermined time period.
U.S. Pat. No. 4,561,064, issued to Bruggen et al., discloses an ultrasonic pulse beam system which is located externally of a motor vehicle and projects a beam to a fixed reflector located on the vehicle. The device includes three threshold timing levels based on the distance between the transducer and the sensed surface, and generates three distinct frequencies in response thereto. To compensate for ambient conditions and dirt on the transmitters, timing intervals can be changed by providing a calibrating input based on actual transit time from the transmitter to the fixed reflector.
U.S. Pat. No. 4,663,712, issued to Kishida, discloses a method of controlling an implement level comprising the steps of averaging the time periods measured by a non-contacting distance sensor, comparing the preset target data with the measured and averaged data, computing difference between them and raising or lowering the implement in accordance with the magnitude of the difference. When averaging the measured data, certain abnormal data which causes erroneous readings is omitted prior to averaging.
Finally, U.S. Pat. No. 4,733,355, issued to Davidson et al., discloses a non-contacting range sensing and control device for controlling the position of a grading implement relative to a datum. The device includes a reference circuit which defines consecutive first, second and third timing intervals for comparing the separation distance between the transducer and a target. A comparison circuit generates error signals if the separation distance falls into the first or third intervals, but generates no error signal if the separation distance is within the limits of the second interval.